Systems and methods for multiple-input multiple-output communications systems

ABSTRACT

In a multiple input multiple output (MIMO) wireless communication system, there is provided a method for error correction. The method includes receiving an original signal in an initial MIMO format, detecting an error in the original signal, and notifying a transmitter of the error detected in the original signal. The method also includes receiving a new retransmitted signal in a first retransmitted MIMO format, different from the initial MIMO format, the new retransmitted signal including at least a fraction of encoded bits of the original signal. The method also includes correcting the original signal by applying the new retransmitted signal to the original signal.

RELATED APPLICATIONS

This application claims benefit from Provisional Application No.61/050,966 filed May 6, 2008, and Provisional Application No. 61/105,923filed Oct. 16, 2008, the contents of which are incorporated in theirentirety by reference.

FIELD OF THE INVENTION

This invention relates to systems and methods for error correction.

BACKGROUND OF THE INVENTION

Wireless communication systems may use either Single Input Single Output(SISO) configurations or Multiple Input Multiple Output (MIMO)configurations. SISO systems include a single antenna at the transmitterand a single antenna at the receiver. By contrast, MIMO systems includemultiple antennae at the transmitter, and multiple antennae at thereceiver. The additional antennae may have features for signaltransmission and reception.

For example, the MIMO system may employ Spatial Multiplexing (SM), whichenables the MIMO system to transmit different signals on differentantennae. SM enables the MIMO system to generally provide greaterthroughput, because more signals are transmitted at a particular timeand/or frequency over the multiple antennae. A MIMO system may transmitadditional SM signals at a subsequent time and/or at a differentfrequency unit. However, when using SM, multiple signals may interferewith each other at the receiver, especially in a highly correlatedchannel.

As another example, the MIMO system may also employ Space Time BlockCoding (STBC), which enables the MIMO system to transmit a space-timecoded signal over multiple antennae during a time interval. In otherwords, the space-time coded signal includes multiple redundant signalsthat are each transmitted over a different antenna. In this way, if oneof the signals is corrupted before it reaches the receiver, a duplicatesignal that is transmitted from a different antenna is available tocorrect or replace the corrupted signal. This is called “diversity.”Moreover, unlike SM, even if the signals are transmitted simultaneously(i.e., at the same time and/or frequency unit), there is no interferenceat the receiver. This is because the space-time coded signal isorthogonal. In other words, the redundant signals which are part of thespace-time coded signal are orthogonal to each other. Therefore, whenthe redundant signals are received at the receiver, there is nointerference.

As yet another example, the MIMO system may also employ Space FrequencyBlock Coding (SFBC), which enables the MIMO system to transmit the samesignal over multiple antennae at a given time, just as STBC. SFBC issimilar to STBC in that it is transmitted orthogonally. However, SFBCdiffers from STBC, in that it sends different signals over differentfrequency sub-carriers, instead of at later points in time. Accordingly,SFBC enables MIMO to send redundant copies of the signal, along withdifferent signals, all at the same time. As demonstrated by theseexamples, the use of MIMO can be beneficial.

At the receiver, the MIMO signal is decoded. There are differentdecoders that may be used to decode the MIMO signal. If the signals areencoded orthogonally, i.e., as STBC or SFBC, there will be nointerference for each signal, and a maximum ratio combining (MRC)detector can be employed at the receiver. However, if the signals arenot encoded orthogonally, i.e., as SM, then the signals may interferewith each other, and a minimum mean square error (MMSE) or maximumlikelihood (ML) detector can be employed at the receiver. Of the threedetectors, MRC is simpler than MMSE, and ML requires the largestcomputational effort.

An optimal decoder to use in any given situation depends on the formatof the transmit signal. The optimal decoder is one that produces adecoded signal that reaches the ML, and known as the best solution. ForSM formatted transmit signals, the optimal detector may be the MLdetector. For STBC/SFBC formatted transmit signals, the optimal detectormay be the MRC detector, as the solution to the MRC is already a MLsolution. The reason that STBC/SFBC may be compatible with the simplerMRC detector, is because the orthogonal encoding of the symbols inSTBC/SFBC cancels the interference among the different signals. However,this is only true given the assumption that a subsequent channel(whether in time or frequency) is slowly or nearly time invariant. Inother words, the assumption is that the subsequent channel does notchange (or changes very little) according to time or frequency.

To increase reliability, both SISO and MIMO systems may employ a HybridAutomatic Retransmission Request (HARQ). With HARQ, the receiverperforms a cyclic redundancy check (CRC) on the received signal. If theresult of the CRC is positive, then the receiver sends anacknowledgement (ACE) to the transmitter. However, if the result of theCRC is negative, the receiver sends a negative acknowledgement (NACK) tothe transmitter. After the transmitter receives the NACK, it retransmitsat least a portion of the original signal to the receiver, so that thereceiver can correct the error in the previously received originalsignal.

When signals are retransmitted using HARQ, the receiver must combine theretransmitted signal with the original signal in order to correct theerror. There are two primary schemes by which to combine these signals.First, the receiver may use bit level combining, whereby the receivercombines the signals at the bit level. Second, the receiver may usesymbol level combining, whereby the receiver combines the signals at thesymbol level. The symbol is a constellation point mapping of acollection of bits. As used here, a symbol is a representation of a unitof data. Whether using bit level combining or symbol level combining, aninitially transmitted signal may be encoded using SM. Moreover,conventional MIMO HARQ systems retransmit the same SM signal when anerror is found in the initially transmitted signal. The retransmittedsignal may be combined with the initially transmitted signal at thereceiver at the bit level. Alternatively, the retransmitted signal maybe combined with the initially transmitted signal at the receiver at thesymbol level, for example with a joint MMSE detector, and the MMSEdetector as discussed above.

In SISO systems, bit level combining and symbol level combining do notdiffer significantly with respect to performance. However, in MIMOsystems, when using the conventional retransmitted SM pattern, usingsymbol level combining improves the combination performance as comparedto bit level combining. This is because a better conditioned equivalentchannel may be obtained before detection when using symbol levelcombining. A channel may be well-conditioned when columns of the channelare substantially orthogonal to each other. In this way, theinterference between SM signals can be fully eliminated.

However, symbol level combining has some drawbacks and restrictions.First, symbol level combining consumes more buffer space in the receiveras compared to other methods of combining in the receiver. Reducingbuffer consumption may be desirable in MIMO systems, especially whenimplementing system on a chip (SoC) design. A second drawback is thatthe retransmitted bits should be aligned in symbol mapping with respectto that of the original transmission. In other words the constellationsignal should be aligned to permit the same constellation points. Third,symbol level combining may consume more computation power than othermethods of combining. Regardless of whether symbol level combining orbit level combining is used at the receiving side, both the initialpacket and the retransmitted packet are customarily sent using the sameMIMO pattern format.

SUMMARY OF THE INVENTION

According to a first aspect of the present disclosure, there isprovided, in a multiple input multiple output (MIMO) wirelesscommunication system, a method for error correction, comprising:receiving an original signal in an initial MIMO format; detecting anerror in the original signal; notifying a transmitter of the errordetected in the original signal; receiving a new retransmitted signal ina first retransmitted MIMO format, different from the initial MIMOformat, the new retransmitted signal including at least a fraction ofencoded bits of the original signal; and correcting the original signalby applying the new retransmitted signal to the original signal.

According to a second aspect of the present disclosure, there is furtherprovided at least one computer-readable medium including programinstructions which, when executed by at least one processor, cause theat least one processor to perform a method for error correction in amultiple input multiple output (MIMO) wireless communication system, themethod comprising: receiving an original signal in an initial MIMOformat; detecting an error in the original signal; notifying atransmitter of the error detected in the original signal; receiving anew retransmitted signal in a first retransmitted MIMO format, differentfrom the initial MIMO format, the new retransmitted signal including atleast a fraction of encoded bits of the original signal; and correctingthe original signal by applying the new retransmitted signal to theoriginal signal.

According to a third aspect of the present disclosure, there is stillfurther provided a multiple input multiple output (MIMO) wirelesscommunication system for error correction, the system comprising:antennae configured to receive an original signal in an initial MIMOformat, notify a transmitter of an error in the original signal, andreceive a new retransmitted signal in a first retransmitted MIMO formatdifferent from the initial MIMO format, the new retransmitted signalincluding at least a fraction of encoded bits of the original signal;and a processor configured to detect the error in the original signaland to correct the original signal by applying the new retransmittedsignal to the original signal.

Additional features of the invention will be set forth in part in thedescription which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. Thefeatures of the invention will be realized and attained by means of theelements and combinations particularly pointed out in the appendedclaims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description, serve to explain the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a 0^(th) transmission in acommunication system.

FIG. 2 is a block diagram illustrating an exemplary MIMO transmitter.

FIG. 3 is a block diagram illustrating an example of bit selection in achase combining mode.

FIG. 4 is a block diagram illustrating an example of bit selection in anincremental redundancy mode.

FIG. 5 is a packet diagram of a 0^(th) transmission and 1^(st)retransmission.

FIG. 6 is a block diagram illustrating a generalized example of a MIMOreceiver receiving signals at the symbol level.

FIG. 7 is a block diagram illustrating a generalized example of a MIMOreceiver receiving signals at the symbol level using joint detection.

FIG. 8 is a block diagram illustrating a generalized example of a MIMOreceiver receiving signals at the bit level.

FIG. 9 is a block diagram illustrating a detailed example of a MIMOreceiver at the symbol level.

FIG. 10 is an error correction flow diagram.

FIG. 11 is a flow diagram illustrating an adaptive MIMO moderetransmission.

FIG. 12 is a block diagram of an exemplary hardware component.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to exemplary embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

As discussed above, whether symbol level combining or bit levelcombining is used at the receiving side, both the initial packet and theretransmitted packet are customarily sent using the same MIMO patternformat. However, in embodiments disclosed herein, the retransmittedpacket can be in any MIMO format, including a MIMO format that isdifferent than the MIMO format used for the initial packet. Generally, alater retransmission MIMO format can be different than an earliertransmission or retransmission. For example, a first (1^(st))retransmission may be of a different format than an initial (0^(th))transmission. Furthermore, a second (2^(nd)) retransmission may be of adifferent format than the initial 0^(th) transmission and/or differentthan the 1^(st) retransmission. Moreover, the MIMO pattern format can beany kind of MIMO encoding scheme, for example, single-user MIMO,multi-user MIMO, open-loop MIMO, closed-loop MIMO, SM, STBC/SFBC andtheir combinations.

As also discussed above, there are different benefits and drawback tousing symbol level combining. Although a better condition number(indicating a better conditioned channel) can be obtained by symbollevel combining, it may be beneficial to use alternative ways to enhancethe performance of bit level combining, in order to avoid the drawbacksof symbol level combining. The alternative ways of enhancing theperformance of bit level combining may include constellationre-arrangement or random pre-coding.

In disclosed embodiments, it may be beneficial to send the retransmittedpacket using a different MIMO format, for example, if the conditionnumber of a retransmission using the original MIMO format is poor. Forexample, when using bit level combining (which generally has a lowercondition number than joint symbol level combining), using a differentMIMO format may be desirable. In this way, the advantages of bit levelcombining may be achieved while compensating for the lower conditionnumber.

FIG. 1 illustrates a block diagram of an initial 0^(th) transmission ina MIMO communication system 100. MIMO communication system 100 mayinclude a signal 102, MIMO transmitter 104, and MIMO receiver 106. Theinitial 0^(th) transmission may be an original transmission of signalss_(A) ⁰ and s_(B) ⁰ as signal 102, from MIMO transmitter 104 to MIMOreceiver 106. MIMO transmitter 104 may include an antenna 108 and anantenna 110. Moreover, MIMO transmitter 104 may include additionalantenna (not shown). MIMO transmitter 104 may send signal s_(A) ⁰onantenna 108, and signal s_(B) ⁰ on antenna 110. In a generalrepresentation s_(m) ^(t) of the signals s_(A) ⁰ and s_(B) ⁰ beingtransmitted, t and m represent the t-th transmission and m-thconstellation signal, respectively. MIMO receiver 106 may include anantenna 112 and an antenna 114. Moreover, MIMO receiver 106 may includeadditional antenna (not shown). MIMO receiver 106 may receive signal 102at antenna 112, and/or an antenna 114, and/or at any additionalantennae. Signal 102 may then be processed by a linear minimum meansquare error (MMSE) estimator MIMO detector 116 to detect thetransmitted signals, and calculate relative soft gains. Block diagram100 may implement the original signal transmission according to thefollowing equation:y ⁰ =Hs ⁰ +n  (1)where y⁰ is the received signal for the 0-th transmission, H is the MIMOchannel between MIMO transmitter 104 and MIMO receiver 106, s⁰ is thesignal 102 for the 0-th transmission, and n is a noise and interferencevector. Equation (1) can also be written in expanded matrix form:

$\begin{matrix}{y^{0} = {{\begin{bmatrix}h_{00} & h_{01} \\h_{10} & h_{11}\end{bmatrix}\begin{bmatrix}s_{A}^{0} \\s_{B}^{0}\end{bmatrix}} + {n.}}} & (2)\end{matrix}$

After the initial 0^(th) transmission is received, MIMO receiver 106 mayperform a CRC check on signals s_(A) ⁰ and s_(B) ⁰. In this example,MIMO receiver 106 may determine that there is an error in the receivedsignal y⁰ which includes s_(A) ⁰ and s_(B) ⁰. Upon making thedetermination that there is an error in the received signal y⁰, inaccordance with HARQ as discussed above, MIMO receiver 106 may send aNACK to MIMO transmitter 104. In response, MIMO transmitter 104 mayretransmit a partial signal of the original packet, for example s_(A) ⁰in the next transmission.

In this example, the retransmission retransmits signal s_(A) ⁰, as s_(A)¹, (with the 1 in the superscript corresponding to the 1^(st)retransmission), and may also transmit new signal s_(C) ⁰ (which is onemember of another packet) as an original transmission. STBC or SFBC maybe used for the 1^(st) retransmission. In these examples y¹ is thereceived signal for the 1^(st) retransmission. If STBC is used, then tworetransmit symbols are sent over two antennae and in two consecutivetime periods, whereas if SFBC is sent, then two retransmit symbols aresent over two antennae and in two consecutive frequency sub-carriers.For example, if using STBC, the equation for the first symbol is:y _(—) _(sym0) ¹ =Hs ¹ +n  (3),which is expanded in matrix form to

$\begin{matrix}{y_{{\_ sym}\; 0}^{1} = {{\begin{bmatrix}h_{00} & h_{01} \\h_{10} & h_{11}\end{bmatrix}\begin{bmatrix}s_{A}^{1} \\s_{C}^{0}\end{bmatrix}} + {n.}}} & (4)\end{matrix}$The equation for the second symbol isy _(—) _(sym1) ¹ =Hs ¹ +n  (5),which is expanded in matrix form to

$\begin{matrix}{y_{{\_ sym}\; 1}^{1} = {{\begin{bmatrix}h_{00} & h_{01} \\h_{10} & h_{11}\end{bmatrix}\begin{bmatrix}{- s_{C}^{0^{*}}} \\s_{A}^{1^{*}}\end{bmatrix}} + {n.}}} & (6)\end{matrix}$In this STBC example, the * denotes a conjugate value, and thearrangement of s_(A) ¹ and s_(C) ⁰ with their respective conjugates inthe two symbols are in accordance with STBC.

MIMO receiver 106 may use several schemes when receiving and detectingan initial 0^(th) transmission and a 1^(st) retransmission. For example,successive interference cancellation (SIC) may be used, in accordancewith which the noise and interference vector n for a signal transmittedon antenna 108 is subtracted from a signal transmitted on antenna 110.As another example, maximum ratio combining (MRC) may be used, inaccordance with which MIMO receiver 106 multiplies a received vectorwith a hermitian (transposed conjugate) of the channel matrix. Thismultiplying may have the effect of boosting signal components andattenuating noise components. As a further example, soft combining maybe used, in accordance with which simple addition is performed tocombine soft information contained in the 0^(th) transmission and 1^(st)retransmission signals.

Disclosed embodiments are not limited to 2×2 MIMO schemes, but mayinclude any number of antennae (e.g., 4×4, 16×16, etc).

FIG. 2 is a block diagram illustrating a MIMO transmitter 200, which maycorrespond to MIMO transmitter 104 from FIG. 1. MIMO transmitter 200 maybe used when transmitting at the symbol level. Alternatively, MIMOtransmitter 200 may be used when transmitting at the bit level. MIMOtransmitter 200 may include a forward error correction (FEC) encoder 202that receives input data 204 and outputs encoded data 206. FEC encoder202 may encode systematic (i.e. information) bits and/or bytes (or anyother collection of data) within input data 204 with parity bits. Theresulting parity bits may be a characteristic of the FEC encoder,regardless of whether all the parity bits are ultimately used. Theparity bits may be used for error correction of the systematic bits atthe receiving end after transmission.

Encoded data 206 may be encoded for error correction at the receivingend. Encoded data 206 may be encoded for an initial 0^(th) transmissionor for a HARQ process.

If transmitter 200 is transmitting an initial 0^(th) transmission, aninitial bits selector 208 may receive encoded data 206. Initial bitsselector 208 may select portions of encoded data 206 for transmission.For example, encoded data 206 may include a systematic bit and twoparity bits. In this case, initial bits selector 208 may select only thesystematic bit and one of the parity bits for transmission. Theseselected bits may include first selected bits 210 outputted by initialbits selector 208. Thus, first selected bits 210 may include fewerparity bits than encoded data 206. This may increase overall sendingcapacity by reducing the total number of bits transmitted. Initial bitsselector 208 may send first selected bits 210 to a first constellationmapper 212.

First constellation mapper 212 may output first selected mapped data214.

First constellation mapper 212 may send first selected mapped data 214to a spatial multiplexing (SM) MIMO encoder 216. SM MIMO encoder 216 mayencode first selected mapped data 214 according to spatial multiplexing(SM) and output an SM signal 218 for transmission to a receiver.

Instead of transmitting an initial 0^(th) transmission, transmitter 200may, in accordance with a HARQ process, retransmit an originallytransmitted signal that was not properly received. Transmitter 200 mayretransmit the originally transmitted signal in accordance with HARQ,such as by chase combining (CC) mode or incremental redundancy (IR)mode.

When transmitter 200 operates in CC mode when retransmitting accordingto HARQ, initial bits selector 208 may send first selected bits 210 to aretransmitted bits selector 220. Retransmitted bits selector 220 mayselect portions or all of the first selected bits 210 forretransmission. Retransmitted bit selector 220 may output these selectedbits as second selected bits 222 to a second constellation mapper 224.Second constellation mapper 224 may be the same as or different fromfirst constellation mapper 212.

Second constellation mapper 224 may be applicable when either symbollevel combining or bit level combining is ultimately used at thereceiving end. For symbol level combining in the receiver, secondconstellation mapper 224 may map the bits and/or bytes within secondselected bits 222 for aligned constellation mappings. In other words,second constellation mapper 224 may align a mapped constellation of thesecond selected bits 222 like a mapped constellation of first selectedbits 210 performed by first constellation mapper 212. Alternatively, forbit level combining in the receiver, second constellation mapper 224 maymap constellation points that are not aligned.

Second constellation mapper 224 may output second selected mapped data226 to a space time blocking code (STBC)/space frequency blocking code(SFBC) MIMO encoder 228. STBC/SFBC MIMO encoder 228 may encode secondselected mapped data 226 according to either STBC or SFBC. STBC/SFBCMIMO encoder 228 may output an STBC/SFBC signal 230 for transmission (asa retransmitted signal) to a receiver.

When transmitter 200 operates in IR mode when retransmitting accordingto HARQ, FEC encoder 202 may send encoded data 206 to retransmitted bitsselector 220. Thus, retransmitted bits selector 220 may, instead ofreceiving first selected bits 210 (as was the case in CC mode), receiveencoded data 206 in IR mode. Retransmitted bits selector 220 may selectportions/multiples of encoded data 206 for retransmission. Retransmittedbit selector 220 may output these selected bits as second selected bits222. Retransmitted bits selector 220 may send second selected bits 222to a second constellation mapper 224. Second constellation mapper 224may output second selected mapped data 226, and send second selectedmapped data 226 to a STBC/SFBC MIMO encoder 228. STBC/SFBC MIMO encoder228 may encode second selected mapped data 226 according to either STBCor SFBC. STBC/SFBC MIMO encoder 228 may output STBC/SFBC signal 230 fortransmission (as a retransmitted signal) to a receiver.

FIG. 3 is a block diagram 300 illustrating an example of bit selectionin CC mode, which may be performed by retransmitted bits selector 220.Retransmitted bits selector 220 may perform selection of a bit, or anyother grouping of data. Block diagram 300 may include base encoded bits302. Base encoded bits 302 may be generated by FEC encoder 202, andincluded in encoded data 206. Base encoded bits 302 may be the resultsof FEC encoder 202 with a 1/3 code rate, such that each group ofsystematic (i.e., information) bits (S) is associated with two groups ofparity bits (P1 and P2). However, any other code rate may be usedalternatively. Base encoded bits 302 may include systematic encoded bits304 and parity encoded bits 306 and 308. Systematic encoded bits 304 maycorrespond to the systematic bits, while parity encoded bits 306 and 308may correspond to the parity bits.

For an initial 0^(th) transmission 309 in CC mode, systematic encodedbits 304 and the parity encoded bits 306 may be selected from baseencoded bits 302, for example, by initial bits selector 208. In otherwords, only systematic encoded bits 304 and parity encoded bits 306 frombase encoded bits 302 are sent on initial 0^(th) transmission 309, as isindicated by shading of their respective blocks. The selection of twoout of the three bit segments (called puncturing) in base encoded bits302 may both decrease the amount of data that needs to be sent from baseencoded bits 302, and that the amount of data that is included in the0^(th) transmission. This may save more spectral efficiency comparedwith non-punctured transmission, thereby enhancing overall systemthroughput. Systematic encoded bits 304 and parity encoded bits 306 maybe transmitted in the initial 0^(th) transmission using SM (as indicatedby their shading). Moreover, systematic encoded bits 304 and parityencoded bits 306 may include, and/or may be included in, first selectedbits 210.

In CC mode, bits for retransmission may be selected from systematicencoded bits 304 and parity encoded bits 306 that were sent with initial0^(th) transmission 309. Examples 310 and 311 are different ways ofusing systematic encoded bits 304 and parity encoded bits 306 forretransmission in CC mode.

Example 310 includes both a 1^(st) retransmission 312 and a 2^(nd)retransmission 314. 1st retransmission 312 may include systematic bitsS, which correspond to systematic encoded bits 304 that were sent ininitial 0^(th) transmission 309. Moreover, 2^(nd) retransmission 314 mayinclude parity bits P1 which correspond to parity encoded bits 306 thatwere sent in initial 0^(th) transmission 309. Bits S and P1 in example310 may be transmitted in 1^(st) retransmission 312 and2^(nd)retransmission 314, respectively, using SFBC/STBC (as indicated bytheir shading).

Example 311 includes both a 1^(st) retransmission 318 and a 2^(nd)retransmission 320. In example 311, bits for the retransmissions mayalso be selected from systematic encoded bits 304 and parity encodedbits 306 that were sent with initial 0^(th) transmission 309. However,example 311 may select individual bits from within S and P1, withoutselecting the entire group of bits, for both 1st retransmission 318 andsecond retransmission 320. The individual bits selected in example 311may be sufficient for error correction at the receiving end. Forexample, the portion of individual bits from systematic bits S of firstretransmission 318, may be sufficient to represent bits S. Moreover,this allows bits from both S and P1 to be selected in both 1^(st)retransmission 318 and 2^(nd) retransmission 320. This may enhance errorcorrection at the receiving end because both the 1^(st) retransmissionand the 2^(nd)retransmission include parity bits from P1. Moreover, thebits selected in 1^(st) retransmission 318 may differ from the bitsselected in the 2^(nd) retransmission 320. Alternatively, the bitsselected in 1^(st) retransmission 318 may overlap with the bits selectedin the 2^(nd) retransmission 320.

FIG. 4 is a block diagram 400 illustrating an example of bit selectionin IR mode, which may be performed by retransmitted bits selector 220.Bit selection in IR mode may be an alternative to bit selection in CCmode, which was described above in FIG. 3. Retransmitted bits selector220 may perform selection of all bits of systematic bits and/or paritybits, or any other grouping of these bits. Block diagram 400 may includebase encoded bits 402. Base encoded bits 402 may be generated by FECencoder 202, and included in encoded data 206. Base encoded bits 402 maybe the result of FEC encoder 202 with a 1/3 code rate, such that eachgroup of systematic (i.e. information) bits (S) is associated with twogroups of parity bits (P1 and P2). However, any other code rate may beused alternatively. Base encoded bits 402 may include systematic encodedbits 404 and parity encoded bits 406 and 408. Systematic encoded bits404 may correspond to the systematic bits, while parity encoded bits 406and 408 correspond to the parity bits.

For an initial 0^(th) transmission 409 in IR mode, systematic encodedbits 404 and parity encoded bits 406 may be selected from base encodedbits 402, for example, by initial bits selector 208. In other words,only systematic encoded bits 404 and parity encoded bits 406 from baseencoded bits 402 are sent on initial 0^(th) transmission 409, as isindicated by shading of their respective blocks. The selection of twoout of the three portions of bits in base encoded bits 402 may improvethroughout by decreasing the number of parity bits sent. Systematicencoded bits 404 and parity encoded bits 406 may be transmitted in theinitial 0^(th) transmission using SM (as indicated by their shading).Moreover, systematic encoded bits 404 and parity encoded bits 406 mayinclude, and/or may be included in, first selected bits 210.

In IR mode, bits for retransmission may be selected from any of bits404, 406, and 408. This is in contrast to CC mode, in which the bits forretransmission may be selected only from bits that were included in theinitial 0^(th) transmission (which in the example of FIG. 4 would besystematic encoded bits 404 and parity encoded bits 406). Moreover,examples 410 and 411 are different ways of using bits 404, 406, and 408in IR mode.

Example 410 includes both a 1^(st) retransmission 412 and a 2^(nd)retransmission 414. 1st retransmission 412 may include systematic bitsS, which corresponds to bits 404 that was sent in initial 0^(th)transmission 409. Moreover, 2^(nd) retransmission 414 may include paritybits P2 that were not sent in the initial 0^(th) transmission. Bits Sand P2 in example 410 may be transmitted in the 1^(st) retransmission412 and 2^(nd) retransmission 414, respectively, using SFBC/STBC (asindicated by their shading).

Example 411 includes both a 1^(st) retransmission 418 and a 2^(nd)retransmission 420. In example 411, bits for retransmission may also beselected from any of bits 404, 406, and 408 from base encoded bits 402.However, example 411 may select individual bits from within S, P1, andP2 without selecting the entire group of bits, for both a 1^(st)retransmission 418 and a 2^(nd) retransmission 420. The individual bitsselected in example 411 may be sufficient for error correction at thereceiving end. For example, the individual bits from bits S of 1^(st)retransmission 418, may be sufficient to represent bits S. Moreover,this allows bits from S, P1, and P2 to be selected in both 1^(st)retransmission 418 and 2^(nd) retransmission 420. This may enhance errorcorrection at the receiving end because both the 1^(st) retransmissionand the 2^(nd)retransmission may include parity bits from P1 and P2.Moreover, the bits selected in 1^(st) retransmission 418 may differ fromthe bits selected in the 2^(nd) retransmission 420. Alternatively, thebits selected in 1^(st) retransmission 418 may overlap with the bitsselected in the 2^(nd) retransmission 420.

As discussed, both FIGS. 3 and 4 include a 1^(st) retransmission and a2^(nd) retransmission. In disclosed embodiments, a constellation orderof the 1^(st) and 2^(nd) retransmission may be different from aconstellation order of the 0^(th) transmission. This may be due to amodulation step-up, in which multiple encoded bits from the 0^(th)retransmission are included in the retransmissions.

Moreover, in example 311 of CC mode and/or example 411 of IR mode, asdiscussed in FIGS. 3 and 4, respectively, retransmitted bits selector220 may use different schemes to select the individual bits forretransmission. For example, bit priority mapping and/or constellationrearrangement may be used for selecting the individual bits forretransmission. Moreover, the number of bits selected for retransmissionmay be a fraction of the number of bits in the initial 0^(th)transmission. The fraction may be a predetermined retransmit fraction (⅓in the examples in FIGS. 3 and 4), that may be predetermined by thetransmitter and/or receiver. Alternatively, the total number ofretransmitted bits may also be larger than the number of bits in theinitial 0^(th) transmission, and this can be accomplished by applying ahigher order of constellation than the initial 0^(th) transmission. Forexample, the total number of retransmitted bits may be a multiple of thenumber of bits in the initial 0^(th) transmission. The multiple may bedetermined by a predetermined retransmit factor, that may bepredetermined by the transmitter and/or receiver. The multiple canitself be a whole number or a mixed fraction. For example, instead ofthe predetermined retransmit fraction being ⅓ in FIGS. 3 and 4, it maybe 5/3.

FIG. 5 is a packet diagram 500 that illustrates an initial 0^(th)transmission 502. Initial 0^(th) transmission 502 may be a packet thatincludes data 504 sent on an antenna 0 at time 0, and data 506 sent onan antenna 1 at time 0. For example, antenna 0 and antenna 1 maycorrespond to antennae 108 and 110 of MIMO transmitter 104. Each symbol(s_(A) through s_(P)) of data 504 and data 506 may be sent at time 0,but on different antennae and/or frequencies. In this example, a CRCcheck is run on initial 0^(th) transmission 502. The CRC check mayreveal an error in initial 0^(th)transmission 502. Based on thepredetermined retransmit fraction, a portion of the symbols s_(A)through s_(P) from data 504 and data 506 may be selected to beretransmitted in an attempt to correct the error in initial 0^(th)transmission 502. The predetermined retransmit fraction may be anynumber 1/N, for N=1, 2, 3, 4, . . . As discussed previously, thepredetermined retransmit fraction may be a whole number or a mixedfraction. In this example, the predetermined retransmit fraction is ¼and, therefore, four symbols out of the original 16 symbols of initial0^(th) transmission 502 are selected for retransmission (s_(A), s_(E),s_(I), and s_(M)). In this example, the selected bits are aligned mappedin constellation mapper 205, but the mapping is not needed if bit levelcombining is applied in the receiver. These symbols are assembled in aretransmission 508, along with new packets 510. By including only aportion of initial 0^(th) transmission 502 in retransmission 508,throughput can be improved by including new packets 510 inretransmission 508.

Retransmission 508 may include data 512 sent on antenna 0 at time 0, anddata 514 sent on antenna 1 at time 0. Retransmission 508 may alsoinclude data 516 sent on antenna 0 at time 1, and data 518 sent onantenna 1 at time 1. The arrangement of symbols s_(A), s_(E), s_(I), ands_(M) in data 512, 514, 516, and 518 may be orthogonal, and consistentwith retransmission using the exemplary technique STBC. Soft informationof the retransmitted symbols s_(A(STBC)), s_(E(STBC)), s_(I(STBC)), ands_(M(STBC)) may then be obtained at the receiver (with the STBC in thesubscript denoting the use of STBC in the 1^(st) retransmission).

FIG. 6 is a block diagram illustrating a generalized example of a MIMOreceiver 600, which may correspond to MIMO receiver 106. MIMO receiver600 may be used when receiving signals at the symbol level.

MIMO receiver 600 may include a retransmission detector 602.Retransmission detector 602 may receive and detect an incomingretransmission 604. Retransmission detector 602 may send a detectedretransmission 606 to a remapper 608. Remapper 608 may remap thedetected retransmission 606 to improve signal quality. Remapper 608 mayperform constellation and/or bit remapping on detected retransmission606. Remapper 608 may send a remapped retransmission 610 to a signalmatcher 612. Signal matcher 612 may determine if there is overlapbetween remapped retransmission 610 and any previously received signal.This overlap can be determined by examining remapped transmission 610.Signal matcher 612 may send a matched retransmission 614 to a symbollevel combiner 616.

MIMO receiver 600 may also include an initial transmission detector 618.Initial transmission detector 618 may receive and detect an incominginitial 0^(th) transmission 620. Initial transmission detector 618 maysend a detected initial 0^(th) transmission 622 to symbol level combiner616. Symbol level combiner 616 may use matched retransmission 614 tocorrect errors in detected initial 0^(th) transmission 622. Thecorrection may be implemented to the extent that matched retransmission614 and detected initial 0^(th) transmission 622 overlap. Symbol levelcombiner 616 may combine portions of matched retransmission 614 withdetected initial 0^(th) transmission 622. Symbol level combiner 616 maysend a combined signal 624 to a FEC decoder 626 for FEC decoding.

FIG. 7 is a block diagram illustrating another generalized example of aMIMO receiver 700, which may correspond to MIMO receiver 106. MIMOreceiver 700 may also be used when receiving signals at the symbollevel. MIMO receiver 700 may use joint detection, unlike MIMO receiver600.

MIMO receiver 700 may include a remapper 702. Remapper 702 may receivean incoming retransmission 704. Remapper 702 may remap incomingretransmission 704 to improve signal quality. Remapper 702 may send aremapped retransmission 706 to a signal matcher 708. Signal matcher 708may also receive an incoming initial 0^(th) transmission 710. Signalmatcher 708 may determine if there is overlap between remappedretransmission 706 and incoming initial 0^(th) transmission 710. Signalmatcher 708 may output a matched retransmission 712 and a matchedinitial 0^(th) transmission 714 to a joint MIMO detector 716. Joint MIMOdetector 716 may operate according to ML. Joint MIMO detector 716 mayinclude a ML Decoder and/or Minimum Mean Square Error (MMSE) detector.Joint MIMO detector 716 may reduce error in matched retransmission 712and/or matched initial 0^(th) transmission 714. Joint MIMO detector 716may send a combined signal 718 to a FEC decoder 720 for FEC decoding.

FIG. 8 is a block diagram illustrating yet another generalized exampleof a MIMO receiver 800, which may correspond to MIMO receiver 106. MIMOreceiver 800 may be used when receiving signals at the bit level,instead of the symbol level. MIMO receiver 800 may include an SM softMIMO detector 802, which may receive an incoming initial 0^(th)transmission 804. Incoming initial 0^(th) transmission 804 may beformatted according to SM. SM soft MIMO detector 802 may send a detectedinitial 0^(th) transmission 806 to a bit matcher 808.

MIMO receiver 800 may also include an STBC/SFBC soft MIMO detector 810,which may receive an incoming retransmission 812. Incomingretransmission 812 may be formatted according to STBC and/or SFBC.STBC/SFBC soft MIMO detector 810 may send detected retransmission 812 tobit matcher 808. Bit matcher 808 may determine if there is overlapbetween detected initial 0^(th) transmission 806 and detectedretransmission 812. Bit matcher 808 may output a matched initial 0^(th)transmission 814 and a matched retransmission 816 and send these outputsto a bit level combiner 818.

Bit level combiner 818 may use the matched retransmission 816 to correcterrors in matched initial 0^(th) transmission 814. The correction may beimplemented to the extent that the matched retransmission 816 and thematched initial 0^(th) transmission 814 overlap. Bit level combiner 818may soft combine portions of matched initial 0^(th) transmission 814with matched retransmission 816. Bit level combiner 816 may send acombined signal 820 to a FEC decoder 822 for FEC decoding.

FIG. 9 is a block diagram illustrating a detailed example of a MIMOreceiver 900, which may correspond to MIMO receiver 106. FIG. 9illustrates processing of signals y⁰ and y¹ to retrieve signals s_(A) ⁰and s_(B) ⁰ in an error free condition. FIG. 9 also illustratesprocessing of STBC HARQ retransmit signal s_(A) ¹, which can be used tocorrect error bearing signals s_(A) ⁰ and s_(B) ⁰. MIMO receiver 900 mayalso include an FEC decoder (not shown) to correct errors in s_(A) ⁰ ands_(B) ⁰.

An STBC equalizer 902 may receive retransmission signal y¹ (as definedin equations 3-6) and may output signals s_(A) ¹ and s_(C) ⁰. STBCequalizer 902 may include an MRC decoder. Signal s_(C) ⁰ outputted fromSTBC equalizer 902 may be a new signal transmitted for the first time,and therefore, is not used for error correction of previously receivedsignals. STBC equalizer 902 may send signal s_(A) ¹ to a soft combiner904 and may also send signal s_(A) ¹ to a hard decision function 906.Hard decision function 906 may quantize the equalized symbols withins_(A) ¹ to their nearest constellation points. Signal s_(A) ¹ may beoutput from hard decision function 906 and sent to an SIC and MRC Block1 (908), which may also receive initial 0^(th) transmission signal y⁰(as defined in equations 1-2). SIC and MRC Block 1 (908) may process thesignals y⁰ and y¹, and then may output s_(B) ⁰ (a component of signaly⁰). The purpose of SIC and MRC Block 1 (908) may be to output anaccurate s_(B) ⁰ that is corrected and error free. SIC and MRC Block 1(908) may extract an accurate s_(B) ⁰ of y⁰ by cancelling the s_(A) ⁰symbol of y⁰ using the s_(A) ¹ symbol of y¹ received from hard decisionfunction 906. The higher diversity (e.g. 4) of s_(A) ¹ may enable SICand MRC Block 1 (908) to cancel the s_(A) ⁰ portions of y⁰. SIC and MRCBlock 1 (908) may operate according to the following equation.y ⁰ −h ₀ ⁰ s _(A) ¹ =h ₁ ⁰ s _(B) ⁰ +n  (7)

SIC and MRC Block 1 (908) may make symbol s_(B) ⁰ available as acorrected symbol. Moreover, SIC and MRC Block 1 (908) may send signals_(B) ⁰ to a hard decision function 910. Hard decision function 910 mayquantize the equalized symbols within s_(B) ⁰ to their nearestconstellation points. Hard decision function 910 may then send s_(B) ⁰to an SIC and MRC Block 2 (912), which may also receive signal y⁰. SICand MRC Block 2 (912) may be similar to SIC and MRC Block 1 (908). Thepurpose of SIC and MRC Block 2 (912) may be to output an accurate s_(A)⁰ that is corrected and error free. SIC and MRC Block 2 (912) mayextract an accurate s_(A) ⁰ of y⁰ by cancelling the s_(B) ⁰ symbol fromy⁰ using s_(B) ⁰ symbol of y⁰ received from hard decision function 910.SIC and MRC Block 2 (912) may send signal s_(A) ⁰ to soft combiner 904.Soft combiner 904 may combine s_(A) ⁰ with s_(A) ¹. The output from softcombiner 904 may include a combination of original error bearing signals_(A) ⁰, which has been processed according to MRC, and retransmitsignal s_(A) ¹, which has been processed according to STBC. The combinedsoft information s_(A(STBC)) ¹+s_(A(MRC)) ⁰ may have a higher tolerancefor noise effect. The signal to noise ratio of the combined softinformation s_(A(STBC)) ¹+s_(A(MRC)) ⁰ may increase as results arecombined. The combined soft information s_(A(STBC)) ¹+s_(A(MRC)) ⁰ mayprovide a more reliable and corrected signal s_(A).

FIG. 10 is a flow diagram illustrating a process 1000 performed by areceiver, e.g., MIMO receiver 106 or 900, for using, as an example, the1^(st) retransmission to determine the correct soft informationcontained in a previously received error bearing signal y⁰. The process1000 starts at 1002. At 1004, the receiver receives an original signalfrom a transmitter, e.g., MIMO transmitter 104. The original signal maybe mathematically represented as:

In the representation of equation 8, the horizontal line represents thespatial domain, while the vertical line represents the frequency domain.In this way, signal [s_(A) s_(B)] is transmitted by two antennas (in thespatial domain) at a first frequency subcarrier, and signal [s_(C)s_(d)] is transmitted by the two antennas at a second frequencysubcarrier.

The original signal from equation 8 may be formatted according to afirst format, such as SM. At 1006, the receiver may run a CRC check onthe original signal. If the CRC check results in no error (1006-No), thereceiver may send an ACE to the transmitter at 1008, therebyacknowledging and error free original signal. Then, at 1009, the processends.

However, if the CRC check does determine the presence of an error in thereceived signal (1006-Yes), the receiver may instead send a NACK to thetransmitter at 1010. At 1012, the receiver may receive a partially ormultiplied retransmitted signal from the transmitter, which may includea fraction of the data from the original signal, or a multiple of thedata from the original signal. Moreover, the retransmitted signal may beformatted according to a second format, different from the first formatof the original signal. For example, the retransmitted signal may beformatted according to STBC and/or SFBC. In SFBC, the retransmittedsignal may be represented as:

In the representation of equation 9, the horizontal line represents thespatial domain, while the vertical line represents the frequency domain.In this way, signal [s_(A) s_(B)] is retransmitted by two antennas (inthe spatial domain) at a first frequency subcarrier, while itsorthogonal signal [s*_(B) −s*_(A)] is transmitted by the two antennas ata second frequency subcarrier. Equation 9 is a partial retransmit of theoriginal signal from equation 8, because only original symbols s_(A)s_(B) are transmitted, while s_(c) and s_(d) are not.

Blocks 1014-1022 are optional steps that may be used for symbol levelcombining. Specifically, blocks 1014-1022 may be used when the initial0^(th) transmission is in SM format and the 1^(st) retransmission and/or2^(nd) retransmission is in STBC/SFBC format. Thus, blocks 1014-1022 aredashed to indicate that they are optional.

At 1014 the receiver may perform SIC processing on the originaltransmission and the retransmission. The SIC may be performed, forexample by SIC & MRC Block 1 (908) in FIG. 9. The SIC processing maysubtract, from the original signal, the interference of theretransmitted signal. Moreover, the purpose of the SIC processing may beto recover an interference free signal s_(B), from the originaltransmission. At 1016 SIC & MRC Block 1 (908) may calculate the resultof this SIC processing as a first degenerated SIMO signal. The firstdegenerated SIMO signal may be represented asy′=s_(B).  (10)At 1018, SIC & MRC Block 1 (908) may apply MRC to this first degeneratedSIMO signal of equation 10 to obtain signal s_(B(MRC)). Soft informationof signal s_(B(MRC)) may then be fed back and subtracted from theoriginal signal (from equation 8) to obtain a second degenerated SIMOsignal. For example, at 1020, SIC & MRC Block 2 (912) in FIG. 9 maycalculate the second degenerated SIMO signal by subtracting s_(B(MRC))from the original signal. The second degenerated SIMO signal mayrepresent the remaining part of the original signal that is also part ofthe retransmitted signal (i.e. s_(A)). The second degenerated SIMOsignal may be determined according to the following equation:y″=s _(A).  (11)At 1022, SIC & MRC Block 2 (912) in FIG. 9 may apply MRC to this seconddegenerated SIMO signal of equation 11 to obtain signal s_(A(MRC)).Further, in addition to, or instead of, using MRC to solve the first andsecond degenerated SIMO signals as outlined above, some embodiments mayuse a MIMO detector to detect signals and enhance performance.

At 1024, Soft Combiner 904 in FIG. 9 may soft combine the softinformation s_(A(MRC)) from the second degenerated SIMO signal with softinformation s_(A(STBC)) from the retransmitted signal. The result of thesoft combining, s_(A(MRC))+s_(A(STBC)), is an attempt by the receiver tocorrect the original packet, which previously failed the CRC check.Alternatively, the soft combiner 904 may soft combine the softinformation of s_(A) ¹ and the soft information of s_(A(STBC)) in bitlevel, in this case, Blocks 1014-1022 are optional steps, as indicatedby dashed lines.

At 1026, a CRC check is run on soft combined signal of a correctedversion of the original packet to determine if there is still an errorin the signal. If there is no error (1026-No), then the receiver maysend an ACE to the transmitter at 1008, and then end at 1009. However,if there is an error in the signal (1026-Yes), the receiver maydetermine if the total number of retransmissions thus far are less thana predetermined number n at 1028. If the number of retransmissions isnot less than n (1028-No), the receiver sends a NACK to the transmitterat 1029, and then may end at 1009. However, if the number ofretransmissions is less than n (1028-Yes), then at 1030 the receiver maysend a NACK to the transmitter. The process then moves back to 1012, andthe receiver receives another retransmission from the transmitter. Insome embodiments, the receiver will send an ACE upon determining thatthere is an error in the signal (1026-Yes), without determining whetherthe total number of retransmissions thus far are less than thepredetermined number n. In those embodiments, the transmitter maydetermine whether the total number of retransmissions would exceed n,and if so, would not send an additional retransmission.

The additional retransmission may include symbols from the originaltransmission, that were not included in prior retransmission. Forexample, an additional retransmission may be represented as:

In the representation of equation 13, the horizontal line represents thespatial domain, while the vertical line represents the frequency domain.In this way, signal [s_(C) s_(D)] is retransmitted by two antennas (inthe spatial domain) at a first frequency subcarrier, while itsorthogonal signal [s*_(D) −s*_(C)] is transmitted by the two antennas ata second frequency subcarrier. Equation 13 is a partial retransmit ofthe original signal from equation 8, because only original symbols s_(C)s_(D) are transmitted, while s_(A) and s_(B) are not (those weretransmitted in the previous retransmission in equation 9).

Using STBC and/or SFBC for retransmission instead of SM may lead tobuffer savings as well as decreased complexity. Disclosed embodimentsare not limited to the use of SM, STBC, and SFBC, and may include otherformats. Moreover, a transmitter may determine which MIMO mode to usefor retransmission.

FIG. 11 is a flow diagram illustrating a process 1100, which may beperformed by, for example MIMO receiver 106. Process 1100 illustrates anadaptive MIMO mode retransmission in which the MIMO mode of theretransmitted signal may depend on feedback sent from the receiver tothe transmitter. Alternatively, a non-adaptive MIMO mode retransmissionmay be used in which MIMO mode of the retransmission may bepredetermined.

Process 1100 begins at 1102. At 1104, the receiver may receive anoriginal transmission. At 1106, the receiver may determine if there is aCRC error in the original transmission. If there is no CRC error(1106-No), then the process ends at 1108. If there is a CRC error(1106-Yes), then, at 1110, the receiver may determine an appropriateMIMO mode for a retransmission. Alternatively, the receiver may notdetermine an appropriate MIMO mode for retransmission. At 1112, thereceiver may send a NACK to the transmitter. The receiver may also sendfeedback to the transmitter which may indicate an appropriate MIMO modefor retransmission. The feedback may describe a channel quality. Forexample, the feedback may include a carrier to interface noise ratio(CINR), rank, and/or a correlation matrix. The rank corresponds to anactual data capacity of the channel. In this way, a transmission canonly be presumed to be error free if an actual MIMO rate does not exceedthe actual data capacity of the channel (i.e., the channel rank.)

The MIMO rate may be a transmission rate of data carried in a MIMOsystem. The MIMO rate may vary according to a MIMO format.Alternatively, the MIMO rate may vary independently of the format. AMIMO rate may vary among different signals transmitted in the MIMOsystem. For example, a MIMO rate of an initial 0^(th) transmission maydiffer from a MIMO rate of a 1^(st) retransmission and/or a 2^(nd)retransmission. Moreover, the MIMO rate of the 1^(st) retransmission maydiffer from the MIMO rate of the 2^(nd) retransmission.

At 1114 the receiver may receive a retransmission from the transmitter.The transmitter may use the feedback to determine which MIMO mode to usefor the retransmission. At 1116, the receiver may correct the originalsignal using the retransmission. Process 1100 may loop back such thatthe receiver may then run a CRC check on the corrected signal at 1106.

There are several MIMO modes that the transmitter can switch among afterreceiving the feedback from the receiver. For example, there may be aswitch from closed-loop MIMO to open loop MIMO and vice-versa. Inanother example, there may be a switch from multi-user MIMO tosingle-user MIMO or vice versa. This switch may occur if only one userreceives a NACK in connection with a 0^(th) initial transmission. Inanother example, there may be a switch from a non-cooperative MIMO modeto a cooperative MIMO mode and vice versa. The cooperative MIMO mode iswhen multiple transmitters transmit different signals to the samereceiver. The non-cooperative MIMO mode is when one transmittertransmits a signal to one or more receivers. The cooperative MIMO modeimproves channel conditions and allows for a different kind of diversitycalled macro-diversity. Accordingly, this may enable reliability to beenhanced by macro-diversity. In another example, there may be a switchfrom non-cyclic delay diversity (non-CDD) MIMO to CDD MIMO, which mayutilize frequency diversity. Frequency diversity is when differentsignals on different channels have different delays. From a frequencypoint of view, these different delays cause a higher fluctuation in thefrequency response. This higher fluctuation corresponds to frequencydiversity. Disclosed embodiments may switch between any combinations ofthe modes disclosed herein.

Moreover, in disclosed embodiments, the retransmitted signal may be sentat an increased or decrease code rate, whether in SM mode, STBC mode, orSFBC mode.

With reference to FIG. 12, each component described herein, e.g., MIMOtransmitter 104 and MIMO receiver 106, may be implemented as a host 1200including one or more of the following components: at least one centralprocessing unit (CPU) 1202 configured to execute computer programinstructions to perform various processes and methods, random accessmemory (RAM) 1204 and read only memory (ROM) 1206 configured to accessand store information and computer program instructions, memory 1208 tostore data and information, one or more databases 1210 to store tables,lists, or other data structures, one or more I/O devices 1212, one ormore interfaces 1214, one or more antennas 1216, etc. Each of thesecomponents is well-known in the art.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. In a multiple input multiple output (MIMO)wireless communication system, a method for error correction,comprising: receiving an original signal in an initial MIMO format;detecting an error in the original signal; notifying a transmitter ofthe error detected in the original signal; receiving a new retransmittedsignal in a first retransmitted MIMO format, different from the initialMIMO format, the new retransmitted signal including at least a fractionof encoded bits of the original signal; correcting the original signalby applying the new retransmitted signal to the original signal; sendingfeedback about the original signal to transmitter, wherein thetransmitter switches the first retransmitted MIMO format from theinitial MIMO format according to the sent feedback, wherein the initialMIMO format and the first MIMO format are chosen from the groupincluding: closed-loop MIMO, open-loop MIMO, multi-user MIMO,single-user MIMO, non-cooperative MIMO, cooperative MIMO, non-CDD MIMO,CDD MIMO, or a combination thereof, and wherein the initial MIMO formatand the first retransmitted MIMO format have a different MIMO rate. 2.The method of claim 1, further comprising: subtracting, from theoriginal signal, a portion of the new retransmitted signal to obtain afirst degenerated signal; subtracting, from the original signal, thefirst degenerated signal to obtain a second degenerated signal; softcombining the portion of the new retransmitted signal with the seconddegenerated signal to correct the error n the original signal.
 3. Themethod of claim 2, further comprising; applying Maximum Ratio Combining(MRC) to obtain soft information for the soft combining.
 4. The methodof claim 1, wherein the initial MIMO format or the first retransmittedMIMO format comprise one or more of Spatial Multiplexing (SM), SpaceTime Block Coding (STBC) Space Frequency Block Coding (SFBC), open-loopMIMO, closed-loop MIMO, single-user MIMO, multi-user MIMO, cyclic delaydiversity (CDD), or cooperative MIMO.
 5. The method of claim 1, furthercomprising: receiving a former retransmitted signal in a secondretransmitted MIMO format, before receiving the new retransmittedsignal; forming a combined signal by combining the former retransmittedsignal with the original signal to attempt to correct the error of theoriginal signal; detecting an error in the combined signal; andnotifying the transmitter of the error detected in the combined signal,wherein the correcting further comprises applying the new retransmittedsignal to the combined signal.
 6. The method of claim 5, wherein: atleast one of the new retransmitted signal or the former retransmittedsignal include a multiple of the encoded bits of the original signal; aconstellation order of the new retransmitted signal is different from aconstellation order of the former retransmitted signal; and a MIMO rateof the new retransmitted signal is different than a MIMO rate of theformer retransmitted signal.
 7. The method of claim 1, furthercomprising: determining the new retransmitted MIMO format according toone or more of a NACK signal or channel quality information, the channelquality information describing one or more of carrier to interferencenoise ratio (CINR), rank, or a correlation matrix.
 8. The method ofclaim 6, wherein at least one of the first retransmitted MIMO format,the second retransmitted MIMO format, the MIMO rate of the newretransmitted signal, the MIMO rate of the former retransmitted signal,or the constellation order is determined dynamically or is predefined.9. The method of claim 1, further comprising: encoding input data at thetransmitter according to forward error correction (FEC), the encodeddata including systematic encoded bits and parity encoded bits;identifying an encoded subset of the systematic encoded bits and theparity encoded bits according to at least the first retransmitted MIMOformat; and selecting the encoded subset to obtain the new retransmittedsignal.
 10. At least one computer-readable medium including programinstructions which, when executed by at least one processor, cause theat least one processor to perform a method for error correction in amultiple input multiple output (MIMO) wireless communication system, themethod comprising: receiving an original signal in an initial MIMOformat; detecting an error in the original signal; notifying atransmitter of the error detected in the original signal; receiving anew retransmitted signal in a first retransmitted MIMO format, differentfrom the initial MIMO format, the new retransmitted signal including atleast a fraction of encoded bits of the original signal; correcting theoriginal signal by applying the new retransmitted signal to the originalsignal; sending feedback about the original signal to the transmitter,wherein the transmitter switches to the first retransmitted MIMO formatfrom the original MIMO format according to the sent feedback, whereinthe original MIMO format and the first retransmitted, MIMO format arechosen from the group including: closed-loop MIMO, open-loop MIMO,multi-user MIMO, single-user MIMO, non-cooperative MIMO, cooperativeMIMO, non-CDD MIMO, or CDD MIMO, and wherein the initial MIMO format andthe first retransmitted MIMO format have a different MIMO rate.
 11. Thecomputer-readable medium of claim 10, the method further comprising:subtracting, from the original signal, a portion of the newretransmitted signal to obtain a first degenerated signal; subtracting,from the original signal, the first degenerated signal to obtain asecond degenerated signal; soft combining the portion of the newretransmitted signal with the second degenerated signal to correct theerror in the original signal.
 12. The computer-readable medium of claim11, further comprising: applying Maximum Ratio Combining (MRC) to obtainsoft information for the soft combining.
 13. The computer-readablemedium of claim 10, wherein the initial MIMO format or the firstretransmitted MIMO format comprise one or more of Spatial Multiplexing(SM), Space Time Block Coding (STBC) or Space Frequency Block Coding(SFBC), open-loop MIMO, closed-loop MIMO, single-user MIMO, multi-userMIMO, cyclic delay diversity (CDD), or cooperative MIMO.
 14. Thecomputer-readable medium of claim 10, the method further comprising:receiving a former retransmitted signal in a second retransmitted MIMOformat, before receiving the new retransmitted signal; forming acombined signal by combining the former retransmitted signal with theoriginal signal to attempt to correct the error of the original signal;detecting an error in the combined signal; and notifying the transmitterof the error detected in the combined signal, wherein the correctingfurther comprises applying the new retransmitted signal to the combinedsignal.
 15. The computer-readable medium of claim 14, wherein: at leastone of the new retransmitted signal or the former retransmitted signalinclude a multiple of the encoded bits of the original signal; aconstellation order of the new retransmitted signal is different from aconstellation order of the former retransmitted signal; and a MIMO rateof the new retransmitted signal is different from a MIMO rate of theformer retransmitted signal.
 16. The computer-readable medium of claim10, the method further comprising: determining the new retransmittedMIMO format according to one or more of a NACK signal or channel qualityinformation, the channel quality information describing one or more ofcarrier to interference noise ratio (CINR), rank, or a correlationmatrix.
 17. The computer-readable medium of claim 15, wherein at leastone of the first retransmitted MIMO format, the second retransmittedMIMO format, the MIMO rate of the new retransmitted signal, the MIMOrate of the former retransmitted signal, or the constellation order isdetermined dynamically or is predefined.
 18. The computer-readablemedium of claim 10, further comprising: encoding input data at thetransmitter according to forward error correction (FEC), the encodeddata including systematic encoded bits and parity encoded bits;identifying an encoded subset of the systematic encoded bits and theparity encoded bits according to at least the first retransmitted MIMOformat; and selecting the encoded subset to obtain the new retransmittedsignal.
 19. A multiple input multiple output (MIMO) wirelesscommunication system for error correction, the system comprising:antennae configured to receive an original signal in an initial MIMOformat, notify a transmitter of an error in the original signal, andreceive a new retransmitted signal in a first retransmitted MIMO formatdifferent from the initial MIMO format, the new retransmitted signalincluding at least a fraction of encoded bits of the original signal; aprocessor configured to detect the error in the original signal and tocorrect the original signal by applying the new retransmitted signal tothe original signal; wherein: the processor is configured to sendfeedback about the original/former retransmitted signal to thetransmitter; the transmitter is configured to switch to the firstretransmitted MIMO format from the initial MIMO format according to thesent feedback; and the transmitter is configured to chose the initialMIMO format and the first retransmitted MIMO format from the groupincluding: closed-loop MIMO, open-loop MIMO, multi-user MIMO,single-user MIMO, non-cooperative MIMO, cooperative MIMO, non-CDD MIMO,CDD MIMO, or a combination thereof, and wherein the initial format andthe first retransmitted MIMO format have a different MIMO rate.
 20. Thesystem of claim 19, wherein the processor comprises: means forsubtracting, from the original signal, a portion of the newretransmitted signal to obtain a first degenerated signal; means forsubtracting, from the original signal, the first degenerated signal toobtain a second degenerated signal; means for soft combining the portionof the new retransmitted signal with the second degenerated signal tocorrect the error in the original signal.
 21. The system of claim 20,wherein the processor is configured to apply Maximum Ratio Combining(MRC) to obtain soft information for the means for soft combining. 22.The system of claim 19, wherein the initial MIMO format or the firstretransmitted MIMO format comprise one or more of Spatial Multiplexing(SM), Space Time Block Coding (STBC) or Space Frequency Block Coding(SFBC), open-loop MIMO, closed-loop MIMO, single-user MIMO, multi-userMIMO, cyclic delay diversity (CDD), or cooperative MIMO.
 23. The systemof claim 19, wherein: the antennae are configured to: receive a formerretransmitted signal in a second retransmitted MIMO format, beforereceiving the new retransmitted signal, and notify the transmitter of anerror detected in the combined signal; and the processor is configuredto: form a combined signal by combining the former retransmitted signalwith the original signal to attempt to correct the error of the originalsignal; detect the error in the combined signal; and notify thetransmitter of the error detected in the combined signal, wherein thecorrecting further comprises applying the new retransmitted signal tothe combined signal.
 24. The system of claim 23, wherein: at least oneof the new retransmitted signal or the former retransmitted signalinclude a multiple of the encoded bits of the original signal; and aconstellation order of the new retransmitted signal is different from aconstellation order of the former retransmitted signal; and a MIMO rateof the new retransmitted signal is different from a MIMO rate of theformer retransmitted signal.
 25. The system of claim 19, wherein theprocessor is configured to determine the new retransmitted MIMO formataccording to one or more of a NACK signal and channel qualityinformation, the channel quality information describing one or more ofcarrier to interference noise ratio (CINR), rank, or a correlationmatrix.
 26. The system of claim 24, wherein at least one of the firstretransmitted MIMO format, the second retransmitted MIND format, theMIMO rate of the new retransmitted signal, the MIMO rate of the formerretransmitted signal, or the constellation order is determineddynamically or is predefined.
 27. The system of claim 19, wherein thetransmitter comprises: means for encoding input data according toforward error correction (FEC), the encoded data including a systematicencoded bits and parity encoded bits; means for identifying an encodedsubset of the systematic encoded bits and the parity encoded bitsaccording to at least the first retransmitted MIMO format; and means forselecting the encoded subset to obtain the new retransmitted signal.